Anaerobic digester systems and methods employ microorganisms to break down biodegradable material in the absence of oxygen. The present invention is of particular significance when the biodegradable material is animal waste from a diary facility, and that example of the present invention will be described herein in detail. However, the principles of the present invention may be applied to other types of biodegradable material and other operating environments.
An anaerobic digester system conventionally comprises a sealed digester tank in which feed material is contained during the anaerobic digestion process. During the anaerobic digestion processes, microorganisms break the feed material down into biogas and digestate. The biogas is removed from the digester tank and may be used as an energy source. The digestate is nutrient rich and may be used as a fertilizer.
When the biodegradable material is animal waste from a dairy facility, the biodegradable material is combined with fluid to facilitate transport and processing of the biodegradable material. Further, solid contaminates (e.g., non-digestible solids such as sand) may be entrained by the fluids used to convey the biodegradable material. The term “raw feed material” will be used to describe the slurry of biodegradable material, fluids, and solid and other contaminates introduced into the digester tank of a digester system.
During the anaerobic digestion process, the feed material within digester tank is largely undisturbed. Heavier solid contaminates within the digester tank, such as sand, thus settle to the bottom of the tank. Settled contaminates can interfere with proper functioning of the anaerobic digestion process and should be removed. Conventionally, contaminates in the raw feed material are periodically removed by emptying the tank and cleaning the settled contaminates from the bottom of the tank.
The need exists for anaerobic digestion systems and methods capable of removing at least a portion of settled solid contaminates from the digesting tank without disrupting the anaerobic digestion process.